This invention relates generally to optical communications systems. More particularly, it provides a novel optical interleaver for multiplexing or de-multiplexing of optical signals.
wavelength division multiplexing (WDM) has emerged as the standard technique to transmit information in fiber-optic networks. This is because as the bandwidth of fiber data increases, electronic sorting becomes increasingly complex, while wavelength routing becomes ever more practical and elegant.
In a WDM system, each optical fiber simultaneously carries many different communications channels in light of respectively different wavelengths. Each channel is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM technology.
WDM systems use components generically referred to as optical interleavers to combine, split, or route optical signals of different channels. Interleavers typically fall into one of three categories, multiplexers, de-multiplexers and routers.
A multiplexer takes optical signals of different channels from two or more different input ports and combines them so that they may be coupled to an output port for transmission over a single optical fiber. A de-multiplexer performs the opposite process, that is, it decomposes an optical signal containing two or more different channels according to their wavelength ranges and directs each channel to a different dedicated fiber. A router works much the same way as a de-multiplexer; however, a router can selectively direct each channel according to control signals to a desired coupling between an input channel and an output port.
FIG. 1 depicts a typical optical interleaver 999 of the prior art as described in U.S. Pat. No. 5,694,233, issued to Wu et al. on Dec. 2, 1997, which is incorporated herein by reference for all purposes. A WDM signal 500 containing two different channels 501, 502 enters interleaver 999 at an input port 11. As used herein, the term xe2x80x9cchannelxe2x80x9d refers to a particular range of wavelengths or frequencies that defines a unique information signal. A first birefringent element 30 spatially separates WDM signal 500 into horizontal and vertically polarized components 101 and 102 by a horizontal walk-off. Component signals 101 and 102 both carry the full frequency spectrum of the WDM signal 500.
Components 101 and 102 are coupled to a polarization rotator 40. The rotator 40 selectively rotates the polarization state of either signal 101 or 102 by a predefined amount. By way of example, in FIG. 1 signal 102 is rotated by 90xc2x0 so that signals 103, 104 exiting rotator 40 are both horizontally polarized when they enter a wavelength filter 61.
Wavelength filter 61 selectively rotates the polarization of wavelengths in either the first or second channel to produce filtered signals 105 and 106. For example, wavelength filter 61 rotates wavelengths in the first channel 501 by 90xc2x0 but does not rotate wavelengths in the second channel 502 at all.
The filtered signals 105 and 106 enter a second birefringent element 50 that vertically walks off the first channel into beams 107, 108. The second channel forms beams 109, 110.
A second wavelength filter 62 then selectively rotates the polarizations of signals 107 and 108, but not signals 109 and 110, thereby producing signals 111, 112, 113, 114 that have polarizations parallel each other. A second polarization rotator 41 then rotates the polarizations of signals 111 and 113, but not 112 and 114. The resulting signals 115, 116, 117, and 118 then enter a third birefringent element 70. Note that second wavelength filter 62 may alternatively be replaced by a polarization rotator 41 suitably configured to rotate the polarizations of signals 111 and 113, but not 112 and 114.
Third birefringent element 70 combines signals 115 and 116, into the first channel, which is coupled to output port 14. Birefringent element 70 also combines signals 117 and 118 into the second channel, which is coupled into output port 13.
As described above, interleaver 999 operates as a de-multiplexer. By operating interleaver 999 in reverse, i.e., starting with channels 501, 502 at ports 13 and 14 respectively, interleaver operates as a multiplexer.
Furthermore, by suitably controlling the polarization rotation induced by rotators 40 and 41, interleaver 999 may be configured to operate as a router.
Polarization rotators used in interleaver 999 contribute to insertion losses in the system. Moreover, these polarization rotators should be zero-order in order to cover a wide bandwidth. Such zero-order plates are however very thin ( less than 0.3 mm), therefore difficult and costly to manufacture.
When interleaver 999 is used as a multiplexer, or in the case where high degree isolation is not required, a single stage interleaver can be used, as shown in FIG. 2. In this prior art, light beam 116 goes through two polarization rotators 40 and 41, while beam 115 travels through only the air. Thus, beam 116 suffers greater loss than beam 115, which causes higher polarization dependent loss (PDL).
What is needed in the art, therefore, is a simpler, more compact and cost-effective optical interleaver that overcomes the shortcomings of the prior art systems and provides more reliable performance.
Accordingly it is a principal object of the present invention to provide a novel wavelength filter module that enables an optical interleaver to be simpler and more compact in structure, thereby providing more reliable performance and lower PDL. It is another object of the present invention to provide methods for constructing optical interleavers that incorporate such wavelength filter modules.
An important advantage of the wavelength filter module of the present invention is that it reduces the number of components required in an optical interleaver, and can be configured in various ways suitable for practical applications. As a consequence, the optical interleaver of the present invention is more compact in design, lower in insertion losses, PDL and cost, while more reliable in performance.
These and other objects and advantages will become apparent from the following description and accompanying drawings.
The present invention provides a wavelength filter module comprising two or more wavelength filters with different predetermined spectral responses, arranged in a xe2x80x9cside-by-sidexe2x80x9d fashion. The wavelength filter module is configured to receive a plurality of input optical signals, where each optical signal passes through only one dedicated wavelength filter. The dedicated wavelength filter selectively passes a predetermined range of wavelengths of the input optical signal with a first polarization and a complementary range of wavelengths of the input optical signal in a second polarization that is orthogonal to the first polarization. Each wavelength filter may be formed from a stacked plurality of birefringent waveplates with each waveplate oriented in a predetermined direction with a different length. The constituent wavelength filters of the module may be configured to have different predetermined orientations, or thicknesses, so as to provide different spectral responses.
The present invention also provides an optical interleaver that employs the wavelength filter module described above. The interleaver generally comprises a first birefringent element that decomposes and spatially separates an input WDM signal containing first and second spectral channels into first and second beams that have orthogonal polarizations. The first and second spectral channels are substantially complementary. A first wavelength filter module is optically coupled to receive the first and second beams, and decomposes the first beam into third and fourth beams with orthogonal polarizations and the second beam into fifth and sixth beams with orthogonal polarizations. The first wavelength filter module is configured such that the third and fifth beams carry the first channel at a vertical polarization, while the fourth and sixth beams carry the second channel at a horizontal polarization. A second birefringent element, optically coupled to the first wavelength filter module, spatially separates the four beams into four horizontally and vertically polarized components, which in turn enter a second wavelength filter module. The third and fourth beams become both vertically polarized but remain spatially separated, and the fifth and sixth beams become both horizontally polarized but remain spatially separated, after passing through the second wavelength filter module. A third birefringent element, optically coupled to the second wavelength filter module, then recombines the third and fifth beams containing the first channel into a first output signal and the fourth and sixth beams containing the second channel into the second output signal. The two output signals are further directed to two output ports.
Each of the first and second wavelength filter modules comprises two wavelength filters with different predetermined spectral responses. The design of each module is such that it advantageously provides the combined function of a polarization rotator and a wavelength filter in an optical interleaver, such as that of the polarization rotator 40 and the wavelength filter 61, or that of the wavelength filter 62 and the polarization rotator 41, in the prior art optical interleaver shown in FIG. 1. The optical interleaver incorporating such a module has fewer components, therefore suffering lower insertion losses, PDL and cost, while providing more reliable performance.
The optical interleaver of the present invention can be configured to operate as a multiplexer, a de-multiplexer, or a router, as depicted in the drawings and the detailed description that follow.
The novel features of this invention, as well as the invention itself, will be best understood from the following drawings and detailed description.